North Korean submarine spotted in international waters

The US has reportedly detected a ramp-up in North Korean submarine activity and tests of 'cold launch' submarine missile ejection systems.

A US defense official told CNN on Monday that 'highly unusual and unprecedented levels' of North Korean submarine activities had been detected, including three 'cold ejection' tests in July.

Cold ejection involves the use of pressurized gas to expel a missile from the launch tube before ignition to reduce the risk of catastrophic damage to the vessel.

The latest cold ejection test took place on land at Sinpo Naval Shipyard on Sunday, the defense official said.

'Highly unusual and unprecedented levels' of North Korean submarine activities had been detected in July. Pictured: Kim Jong Un boards a North Korean submarine in 2014

North Korea has conducted three 'cold ejection' tests of submarine launch tubes in July alone. Pictured: Kim Jong Un peers through the periscope aboard a North Korean sub in 2014

North Korea also engaged in 'unusual deployment activity' earlier this month, with a Romeo-class submarine patrolling 100 kilometers out to sea off the coast of Japan, much farther than usual, the official said.

North Korea is believed to have about 70 submarines in its fleet, but most are older and unable to fire missiles.

The regime's newest and largest submarine, the diesel powered Sinpo-class, is the only one believed to have ballistic missile launch capability.

The Sinpo-class is believed to have a range of 2,800 kilometers, less than half the distance from North Korea to Hawaii.

There is only one confirmed Sinpo-class sub (also called Gorae, or whale) in the North Korean fleet.